When Two Worlds Clash
by madi-carstairs
Summary: Signals of large demonic activity in Central Park send the Shadowhunters of the New York Institute out hunting. Little did they know that the demigods of Camp Half-Blood are out to investigate as well. The worlds of the Nephilim and Half-Bloods are about to collide...
1. Chapter 1

Chapter 1: Beginnings

The Institute was strangely peaceful. For all that had been going on in the Shadowworld, the past few days for the New York Shadowhunters had been remarkably calm. There were few demonic attacks, no one had gotten injured, Jace's fiery condition was gone, and Sebastian was keeping an extremely low profile. Everything was perfect. It was so quiet that—

"IT'S READY!"

The peacefulness, it seemed, had come to an end with Isabelle's voice. Jace and Clary, relaxing on the floor of the library, visibly shuddered.

"If she thinks for one second that I am going to eat whatever monstrosity she has cooked up, she belongs in a mundane psycho ward," Jace muttered, pulling his girlfriend closer to him.

"You think it's too late to run away to Taki's?"Clary asked desperately.

"It's never too late. Let's go." Jace stood up, pushing his golden hair out of his equally golden eyes and reaching a hand down to Clary. She smiled and took it. The couple ran out the door of the room, laughing, and were met in the hallway with an angry Isabelle holding a wooden spoon in her hand.

"There's no need to "escape" this time, guys," she said, putting her hand on her hip. "I followed the instructions just as they were written."

Clary blushed. "We weren't trying to _escape_, Izzy, we were, um, just… going on a date."

Isabelle rolled her eyes. "Please, Clary. You two don't go on _dates_, you just stay here and make out. Now, you are going to eat the food I made, and you are going to like it." She turned around, heading back toward the kitchen.

"Angel help us…" Jace whispered, squeezing Clary's hand.

"What was that?" Isabelle spun around, narrowing her eyes at Jace.

"Nothing."

Just as Izzy was about to whack Jace with her spoon, the elevator door at the end of the hall opened loudly, revealing an extremely panicked Alec.

"Guys, we have to go _right now_. I was just walking by Central Park, and picked up HUGE demonic signals—more than I've seen in a long time."

Gear was thrown on hastily, and the Shadowhunters were out the door, clueless of who they would soon meet.

* * *

It was an average day at Cap Half—Blood. The Apollo kids were practicing archery with the Hunters, the Ares cabin was practicing sword fighting, the Demeter cabin was chasing after Connor and Travis Stoll for pranking them, and monsters were howling in the woods. The Seven of the prophecy were simply resting after lulling Gaea to sleep in Rome. Jason, Frank and Hazel had chosen to stay with their Greek questmates and serve as ambassadors from New Rome.

Percy and Annabeth were relaxing on the beach, her head in his lap, talking about simple things. Since they had fallen into Tartarus, the two had become almost inseparable. She even slept in his cabin (to the endless teasing by the children of Aphrodite).

"Percy, it's impossible for a dolphin and a lion to have a child, so it really doesn't matter what the would be named."

"Forget about that! Just choose which sounds better, Lolphin or Dion?"

"Percy, are you serious…"

"Yes!"

Annabeth sat up, putting her arms around his neck.

"Why do I put up with your insanity?" she whispered in his ear.

"Because you love me," he said with a goofy grin, closing the distance between their lips. He kissed her softly, gently wrapping his arms around her waist, pulling her onto his lap. She ran her fingers through his hair, sighing into his mouth…

"Woah... Sorry guys."

Their moment was interrupted by Jason and Piper. The two were standing close together, holding hands, with sheepish expressions on their faces.

"It's fine, Superman. What's up?" Percy asked, standing and helping Annabeth up.

"We were just flying over Central Park," Piper began, "and we saw a ton of monsters. I haven't seen that many since the war. I have no idea why they're all gathering now."

Annabeth glanced at her boyfriend. "Can you call Blackjack?"

"Sure."

"We'll take Tempest; it'll be quicker," Jason said.

Annabeth narrowed her gray eyes, staring at her three companions. "Be careful, everyone. We've made it through a lot, but still, watch out for surprises."

The demigods mounted their horses and flew off, unaware of who they would meet.


	2. Chapter 2

Chapter 2: Battlefield Encounters

As soon as the Shadowhunters had seen the mass of demons at the park, they knew that Alec wasn't exaggerating. They immediately leaped into battle, sending demon after demon back into whatever hellish realm it had come from. Isabelle ran after a lizard-like monster that reeked of rot and disease and made it fall, howling, with a quick snap of her whip before moving on to the next demon. Alec ran through the horde, notching arrow after arrow and shooting them with perfect aim into the hearts of the beasts. Jace and Clary fought near each other, seraph blades glowing, slashing and stabbing, bringing down all the hideous creatures that dared to come near them. Alec turned to see his _parabatai_ shout in anger: an Oni demon had jumped on top of Jace and lunged for his throat. He quickly fired at it, satisfied when it crumbled into ash and blew away. Jace grinned at him, flipped over an Iblis demon, stabbing downward in midair, and joined Isabelle in fighting a twelve-foot tall skeleton-like creature.

Yet more kept coming; the demons seemed to be nearly infinite in number. The Nephilim were still outnumbered at least twenty or one. Clary, looking up after dispatching a Ravener—making her feel déjà vu— felt a sense of hopelessness wash over her. There were just too many of them to fight without making the odds of death too high to risk. She heard Alec cry out in pain—whirling around, she saw that a spider demon had bit him in the leg, which his sister quickly killed. She was about to shout out for them to retreat when two dark shapes descended from the sky and landed on the sidelines of the battle—were those horses? Just as Clary started to wonder if she had inhaled a bit too much demon blood, four teenagers—two boys and two girls—jumped of the horses, brandishing metallic swords and daggers and rushed into the melee. They were just as effective warriors as the Shadowhunters, fighting like they'd been doing it all their lives. The boy near Jace looked like he had a hurricane around him, while the other one's sword flashed like lightning as he stabbed a snarling hellhound. The girl near Isabelle appeared to be glowing pink.

Yes, she had _definitely_ inhaled too much demon blood.

With the aid of the strange fighters, the Nephilim made quick work of the remaining demons. After all that remained were piles of ash and dust, the two groups turned to face each other, Isabelle supporting her limping brother. Clary took another look at the four. The hurricane boy was handsome, with disheveled black hair and sea-green eyes. He held hands with a pretty girl with blond hair who looked like a typical California girl—except for her calculating gray eyes. The boy with the flashing gold sword had blond hair and bright blue eyes, standing close to the ridiculously beautiful girl with choppy brown hair and eyes that changed color. They looked trained and athletic enough to be Nephilim themselves, except for the fact that—

"You don't have any Marks," Jace said almost accusingly.

Whatever the others had expected them to say, it clearly wasn't that. They all looked at each other strangely. The California girl narrowed her eyes.

"You mean those tattoos on your arms? Who the hell are you, anyway?"

"I could ask you the same thing," he retorted, glaring. "Are you warlocks, or some of the fey?"

"What? Dude, we're demigods. You know, like Hercules and Perseus?" The black—haired guy said, telling them about Camp and half—blood life.

"So, you're saying that you all are the super-powerful kids of some thousand-year-old Greek gods," Isabelle said dryly.

"Well, Roman, too," the blond boy said, ignoring her tone. "I'm Jason Grace, son of Jupiter, Praetor of the Twelfth legion." Well, that explained the flashing sword.

"Piper, daughter of Aphrodite," the kaleidoscope-eyed girl said with reluctance.

"Percy Jackson, son of Poseidon."

"Annabeth Chase, daughter of Athena. Now, once again, you are?"

"We're Nephilim. Shadowhunters," Alec said while Isabelle drew an _iratze_ on his hand.

"You hunt shadows?" The black-haired guy—Percy—said, sounding confused.

"We fight demons. It's not a literal title," Jace said, rolling his eyes, quickly explaining the story of Jonathan Shadowhunter and the purpose of the Clave. "I'm Jace Herondale, this is Clary Fairchild, and they're Alec and Isabelle Lightwood."

Annabeth looked like she was about to ask more questions, but Isabelle interrupted her.

"We can talk more later, but the healing runes aren't working on Alec. We need to get him back to the Institute— I called Magnus; he said he'd meet us there."

The demigods looked at each other, having a silent conference, before Annabeth nodded.

"Alright. This better not be a trap."

**A/N: Thank you for reading! Just to avoid confusion, this is set after HoH and before CoHF (I'm ignoring the fact the Malec broke up). Updates will probably be every 2—6 days. **

**Thanks again and please review! —Madi **


	3. Chapter 3

Chapter 3: The Institute

The Shadowhunter girls—Clary and Isabelle, as Jace had said—walked in front of the group, leading the way to what they had called "the Institute". Jace was slightly behind, helping Alex—no, _Alec_—limp along_. _Percy, Annabeth, Jason and Piper huddled in a little group in the rear, weapons at the ready (hey, you could never be too careful).

"This is so weird," Piper whispered, as everyone nodded.

"It actually makes sense," Annabeth said, her eyebrows furrowed together in concentration. "I mean, there are several other religions and mythologies worldwide. It's only logical that the others have their own…protectors, if you will."

Jason sighed. "What's next? Egyptians?" Percy remained strangely silent.

"Maybe. Who knows how many other groups there are," Annabeth said grimly. They all contemplated that slightly disturbing notion for a minute, before hearing Jace's annoyed voice ahead of them.

"…damn it, Alec, what've you been eating? _Bricks_?"

"Well, I'm _sorry_ I was trying to watch all of your backs." Alec's voice sounded strained.

"I'm going to go help," Percy decided. "Jason, could you IM Leo and tell him to come over here when he can?"

He nodded. "Sure, man."

Percy ran up to the Shadowhunters. "Want a hand?"

"Sure," Jace said. "This guy doesn't look it, but he's pretty damn heavy." Percy put Alec's left arm over his shoulder, just like Jace was doing with his right. Only then could he see the extent of the wound on his leg.

It was a good inch or more deep, just above his knee, and was covered in thick, boiling black sludge—definitely venom. Percy wasn't a medic, but he knew that it probably wasn't a good thing when your blood was bubbling.

"Gods of Olympus, what happened?"

Alec grimaced. "Spider demon."

It was Percy's turn to grimace, remembering Arachne. "Yeah, my girlfriend and I had to fight something like that. Not pleasant. Is there someone at the mysterious Institute of yours that'll heal it?"

The Shadowhunter blushed. "My boyfriend's a warlock, and he's really good at healing spells. Plus, we have the Silent Brothers. They're like monks, but they creep everyone out."

Jace shrugged, or at least tried to. "Brother Zachariah's alright. But the rest of them, well… not everyone can have my angelic looks."

"What's a warlock? Like, a wizard, or something?" Percy asked, confused.

"We'll explain later," Jace continued. "And here we are. This is the Institute."

They had stopped in front of what looked like an old Gothic cathedral, nothing supernatural or unusual. Looking over his shoulder, Percy saw Annabeth staring at the building, thinking about the architecture, no doubt. He grinned.

"So, do we go inside now?" Jason asked.

Clary smiled. "Well, you can see the building, so you're definitely not just crazy mundanes."

"Glad you had so much faith in us," Piper muttered. They approached the building, walking up the steps just as a brown-haired boy in a red shirt popped out of nowhere, making almost everyone jump. He was extremely pale, even for New York, and moved remarkably fast.

"Simon!" Isabelle grinned, stalked over and gave him a hug.

"Hey, Izzy." He looked up, apparently noticing the four people he'd never met before with the rest of his friends. "Um, hey, guys. I'm Simon."

The half-bloods introduced themselves quickly. "Are you a Shadowhunter, too?"Annabeth asked.

"Nah. I'm a vampire."

"He just wishes he was as cool as us," Jace interjected.

"Wait, so vampires exist?" Jason asked.

"And weirder stuff," Clary admitted. "Simon, I'll fill you in right now. We just really need to get Alec's leg fixed up. Is Magnus here yet?"

"Talking about me?" A tall, thin, Asian young man walked up behind them. He was tan, with spiked hair dusted in glitter, wearing leather pants and a glittery yellow tank top. When he blinked, Percy realized he had the slit-pupiled eyes of a cat.

"Thank God," Isabelle exhaled. "My brother got himself wounded—_again_."

The man—Magnus—hurried over to assess the bite, frowning. Alec bit his lip.

"Hey," he said nervously. Magnus rolled his eyes.

"You stupid Nephilim," he said with affection. He gestured towards the door of the Institute, his fingers trailing blue sparks. "Alright, everybody inside."

* * *

They had stopped in a den-like room on the left wing of the building. Clary and Isabelle were talking to Simon, probably telling him what was going on, with Jace's arm around the red-head's waist. Jason and Piper relaxed on a love seat, talking to Percy. Annabeth sat on her boyfriend's lap, watching Magnus chant, his hands glowing blue, and place them on Alec's wound, making it fade away. Alec murmured a thanks as Magnus sat next to him on the couch, wrapping an arm around his shoulders. The conversations died down as the other Shadowhunters and Simon took a seat.

"You're demigods, aren't you?" Magnus asked, glancing at them through his feline eyes.

Annabeth leaned forward, narrowing _her_ eyes. "How'd you know about us?"

"I'm the High Warlock of Brooklyn, darling. It's my job to know about all of the strange beings near me."

Alec scrunched his eyebrows together. "Why didn't you tell us earlier?"

"I made an oath to never reveal it." The warlock didn't seem to want to divulge any more than that. The room entered in an awkward silence for a minute.

"Would you mind explaining more about the Shadowhunter thing? I'm a bit confused," Piper asked with an apologetic smile, breaking the quiet.

"What do you want to know?" Clary asked, leaning against Jace.

"The backstory," Percy suggested. Everyone looked at him strangely. "Ok, so demigods are all over Greek mythology, and seeing that huge library on the way in, I bet you know all about it. You know that we're the children of the gods and mortals. We don't know anything about you guys; we don't even know what Shadowhunters _are_, besides the demon-hunting. I think we should start from the literal beginning."

"That was actually an intelligent comment, Seaweed Brain," Annabeth said with a smirk, nudging his shoulder.

"Glad you have so much faith in me," he muttered under his breath. She kissed him on the cheek.

"The story starts with a man named Jonathan Shadowhunter," Jace began. "In the eleventh century, he noticed that mankind was losing the battle against demons. With the aid of a warlock, he summoned the Angel Raziel and begged him to help. The Angel saw that his intentions were pure and agreed. Raziel mixed his blood with that of Jonathan Shadowhunter's in a cup made of _adamas_—the material of heaven—and told Jonathan to drink it himself and give it to his followers to drink as well, changing them. The result was the first Shadowhunters. The cup he drunk out of, the lake the Angel was called from, and a sword given to Jonathan Shadowhunter became known as the Mortal Instruments."

"Wait, so you're all Nephilim just because a thousand years ago, your ancestors drank Angel and human blood?" Annabeth asked.

"Shadowhunter blood is dominant," Alec replied.

"What about vampires and warlocks?" Jason asked.

"Vampires, werewolves, warlocks and faeries are all Downworlders," Isabelle explained. "They're partially demonic, but have souls, unlike demons. Vampires"—Simon grinned—"and werewolves are humans who've contracted demon diseases, faeries are half-demon, half-angel, and are pretty, but have an evil nature, and warlocks—"

"Half-demon, half-human," Magnus cut in. "And of course, we can do magic. All warlocks have some feature that marks them as not fully human."

"You know I love your eyes," Alec whispered, leaning closer to his boyfriend.

"And the tattoos?" Piper asked.

Clary smiled. "They're runes, or Marks. They give us special abilities—increased speed, strength, night vision, etc. but can turn mundanes, or humans, insane, or into monsters."

"Pleasant," Percy muttered. "So, all of you are, like, helpers of mankind? You all work together?"

The others shared a look.

"Most of us," said Jace, frowning as if remembering something exceedingly unpleasant. "There was a man named Valentine…"

* * *

**A/N: I'm so sorry I haven't updated! I was an idiot and forgot my laptop charger when I was at camp, so I made this an extra long chapter. Thanks for reading! Tell me what you think! Leo and more characters next chapter:)**


	4. Chapter 4

**A/N: I'm sorry for the late update—school is killing me. But, at last, here is the next chapter!**

* * *

Chapter 4

"Valentine Morgenstern was a Shadowhunter, like us," Jace began, resting his head on his hand. "When he was a teenager, he formed a group called the Circle whose goal was to reform the laws of the Clave—our government—and better Shadowhunter society. Then, after his father was killed by a werewolf, his goals changed. He wanted to eradicate all Downworlders as well as all demons, claiming that, as they were part demonic, they did not belong in this world. He attempted to trap Downworlder representatives and murder them, but his wife informed others of the plan and it was the Circle that was defeated. We all thought he died, but he came back this year and pretty much tried to kill everyone who didn't support him. Long story short, he died." The room was silent.

"So… The guy's pretty much Shadowhunter Hitler," Percy spoke up.

Magnus smiled without humor. "Just about. It wasn't exactly a great time, back when the Circle was around. Downworlders were afraid to step outside for fear that they'd be murdered. Before that, we weren't given near equal status with the Clave. But after we fought against Valentine with the other Shadowhunters, we were given seats on the Nephilim's Council, which is a step forward."

"Equality. That's always a step forward. And since Valentine is dead now, you all must be pretty peaceful," Annabeth commented.

Clary laughed. "I wish. Valentine was, um, my dad—"

"And we thought our parents were bad role models," said Piper, wide-eyed.

"—though I didn't know it until recently, since I was living with my mom. She made me think I was just an ordinary mundane. Valentine had experimented on me and Jace—who he had kind of adopted—with angel blood to make us better fighters." Clary took a deep breath. "He experimented on my older brother, who I didn't know existed, with demon blood. The demon blood made him a stronger warrior, but it took away all of his humanity, morals and emotions." She paused, as if remembering something extremely unpleasant. "He's found a way to make dark Shadowhunters using demon blood, and he's going to burn down the world and give it to the demons."

"Not a guy you want to invite to a party, then," Percy muttered. "Does he have a name, or do you just call him Evil Brother or Demon Dude?" Annabeth swatted his arm.

"His name is Jonathan, but everybody calls him Sebastian," Isabelle said. "It's—"

"A long story. I get it," Jason said, looking troubled. "Of course, with our luck, we have to deal with both Gaea and this psycho."

Alec leaned forward. "Gaea? As in, the Earth goddess?"

Simon snorted. "You guys have to fight the Earth? Dude, that sucks."

"Yep. First we had to deal with Kronos, who recruited an army of demigods, and then, BOOM! Jason and I get our memories wiped and switch places, we learn that the Romans and Greeks have stayed separate for centuries but have to get together to stop Mother Nature, and we go on a huge quest to close the doors of the Underworld. Meanwhile, Rome tries to kill us because of an innocent misunderstanding (which we finally explained), Annabeth and I fall into freaking _Tartarus_, we kill some giants, and Piper charmspeaks Gaea into going back to sleep, but our camp director, Chiron, says that'll only last a few months." Percy said rapidly.

"Wow," Clary laughed. "Well, your lives don't sound any less exciting than ours. How did you…"

She stopped talking when the air began to shimmer like a mirage in the middle of the room. An image formed in the air of a short Latino boy with a mischievous smile who looked like he was building something out of pipe cleaners and rubber bands. He wore the same orange shirt as Percy, Piper and Annabeth and had a tool belt looped over his jeans.

"Hey, Leo," Piper grinned.

"Hey, guys! And, um, people I don't know. I'm outside the Institute thingy—it looks like a church right?—but the doors won't open and I don't think it would be a good idea to break them."

"I'll go open them," Jason said, and wiped his hand through the mist, making the other boy's image disappear.

"I'll come with you," Jace said, smirking. "We don't want Gladiator here getting lost."

Jason rolled his eyes as they left to the Institute entrance.

"What was that?" Isabelle asked, her eyebrow raised.

"An Iris message. The goddess of the rainbow allows us to communicate to each other through them. All you need to do is create a rainbow, throw in a golden drachma, and say the name of the person you want to contact. It's helpful because we can't use phones—they attract monsters," Annabeth explained.

"We have something like that—it's like the hologram from Star Wars, and they call it a Projection," Simon said enthusiastically.

"What's Star Wars?" Alec asked, looking confused.

"I'll explain later," Magnus said. "And the scrawny Latino who, due to the tool belt, I'm guessing is a son of Hephaestus is…?"

"That's Leo. He's an, ah, interesting character," Piper said with a smile. "And yeah, he's the Hephaestus cabin counselor." They could already hear Leo's excited voice echoing throughout the halls of the Institute.

"…so cool dude! I can't believe you guys have been here all this time and we never knew! That's freakin' insane! And you've had to deal with crazies trying to take over everything too! It's a good thing none of us messed up, huh…"

"Actually, I've always wanted to be killed or oppressed by a psychopath," Jace said sarcastically.

"Yeah, I'm glad we all survived."

They reached the room where the Nephilim and half-bloods were resting. Jason and Jace went to sit back down, while Leo looked around curiously.

"Uh, hi. I'm Leo Valdez, son of Hephaestus, Supreme Commander of the Argo II, and currently single," he said with a wink at Isabelle, who rolled her eyes while taking Simon's hand. "And you guys are…?"

Everyone quickly introduced themselves and Leo perched on the armrest near Jason and Piper.

"We already told him everything," Jason began, just as a woman of about forty, staring at a scroll, walked into the room. She had long black hair pulled back in a bun and shadows under her blue eyes. She looked a lot like…

"Mom," Isabelle spoke up. "Did you need us?"

The woman looked up, startled. "Oh… I didn't notice where I was going. I must have taken a wrong turn—I was headed towards the library." She narrowed her eyes, seeming to just notice the strangers in orange shirts sitting around the room. "Who are your friends?"

"They're Shadowhunters from the Institute in Boston," Jace lied smoothly. "They were visiting and just dropped by."

The woman nodded, apparently not registering the fact that they had no runes. "I'm Maryse Lightwood, head of the Institute. I apologize for not greeting you; I was just speaking with the Silent Brothers."

Alec sat up. "Did something happen, Mother?"

"The Brothers just told me. Eleven murders of Shadowhunters, Downworlders, and what seem to be mundanes have been reported. Their throats and wrists were all slit and their blood was drained. A note saying _Erchomai_ was near each of the bodies, which makes Jonathan Morgenstern seem to be the culprit." Maryse took a deep breath, looking at the wide-eyed stares of those in the room. "The strange thing, however, was that all of the bodies were partially stuck in the ground—not like Jonathan buried them there, but like the Earth grew up around them."

* * *

**A/N: DUN, DUN, DUUUUUUN! Anyway, we have Leo and Mayrse show up in this chapter—if you have any suggestions of characters you'd like me to put in the story, just let me know and I'll see if I can fit them in. I'm thinking maybe Romans next chapter? **

**Thanks for reading! R & R!**


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